R. S. Ginzburg, S. S. Khidekel, G. Y. Knyazeva, A. A. Sankin a course in modern english

§ 51. Semantic Similarity
of Morphemes
and Word-Families

Lexical groups composed of words with semantically and phonemically identical root-morphemes are usually defined as word-families or word-clusters. The term itself implies close links between the members of the group. Such are word-families of the type: lead, leader, leadership; dark, darken, darkness; form, formal, formality and others. It should be noted that members of a word-family as a rule belong to different parts of speech and are joined together only by the identity of root-morphemes. In the word-families discussed above the root-morphemes are identical not only in
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meaning but also in sound-form. There are cases, however, when the sound-form of root-morphemes may be different, as for example in sun, sunny, solar; mouth, oral, orally; brother, brotherly, fraternal, etc.; their semantic similarity however, makes it possible to include them in a word-family. In such cases it is usual to speak of lexical suppletion, i.e. formation of related words of a word-family from phonemically different roots. As a rule in the word-families of this type we are likely to encounter etymologically different words, e.g. the words brother and mouth are of Germanic origin, whereas fraternal and oral can be easily traced back to Latin. We frequently find synonymic pairs of the type fatherly— paternal, brotherly—fraternal.
Semantic and phonemic identity of affixational morphemes can be observed in the lexical groups of the type darkness, cleverness, calmness, etc.; teacher, reader, writer, etc. In such word-groups as, e.g. teacher, musician, etc., only semantic similarity of derivational affixes is observed. As derivational affixes impart to the words a certain generalised meaning, we may single out lexical groups denoting the agent, the doer of the action (Nomina Agenti)—teacher, reader, etc. or lexical groups denoting actions (Nomina Acti)—movement, transformation, etc. and others.